Enhancement of adsorption efficiency of crystal violet and chlorpyrifos onto pectin hydrogel@Fe3O4-bentonite as a versatile nanoadsorbent

The magnetic mesoporous hydrogel-based nanoadsornet was prepared by adding the ex situ prepared Fe3O4 magnetic nanoparticles (MNPs) and bentonite clay into the three-dimentional (3D) cross-linked pectin hydrogel substrate for the adsorption of organophosphorus chlorpyrifos (CPF) pesticide and crystal violet (CV) organic dye. Different analytical methods were utilized to confirm the structural features. Based on the obtained data, the zeta potential of the nanoadsorbent in deionized water with a pH of 7 was − 34.1 mV, and the surface area was measured to be 68.90 m2/g. The prepared hydrogel nanoadsorbent novelty owes to possessing a reactive functional group containing a heteroatom, a porous and cross-linked structure that aids convenient contaminants molecules diffusion and interactions between the nanoadsorbent and contaminants, viz., CPF and CV. The main driving forces in the adsorption by the Pectin hydrogel@Fe3O4-bentonite adsorbent are electrostatic and hydrogen-bond interactions, which resulted in a great adsorption capacity. To determine optimum adsorption conditions, effective factors on the adsorption capacity of the CV and CPF, including solution pH, adsorbent dosage, contact time, and initial concentration of pollutants, have been experimentally investigated. Thus, in optimum conditions, i.e., contact time (20 and 15 min), pH 7 and 8, adsorbent dosage (0.005 g), initial concentration (50 mg/L), T (298 K) for CPF and CV, respectively, the CPF and CV adsorption capacity were 833.333 mg/g and 909.091 mg/g. The prepared pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent presented high porosity, enhanced surface area, and numerous reactive sites and was prepared using inexpensive and available materials. Moreover, the Freundlich isotherm has described the adsorption procedure, and the pseudo-second-order model explained the adsorption kinetics. The prepared novel nanoadsorbent was magnetically isolated and reused for three successive adsorption–desorption runs without a specific reduction in the adsorption efficiency. Therefore, the pectin hydrogel@Fe3O4-bentonite magnetic nanoadsorbent is a promising adsorption system for eliminating organophosphorus pesticides and organic dyes due to its remarkable adsorption capacity amounts.

Instruments. The chemical solvents and reagents applied in this work, viz., FeCl 3 .6H 2 O, FeCl 2 .4H 2 O, distilled water, ammonia solution (25%), acetone, pectin, calcium chloride, methanol, sodium hydroxide, bentonite, and ethanol were provided by Sigma-Aldrich Company, USA. Based on the preparation process of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent, some analytical and spectroscopic experiments have been taken to authenticate the structural features in every preparation stage as follows. Fourier-transform infrared (FT-IR) spectra by Shimadzu FTIR-8400S model, Japan, apparatus using KBr pellet. The nanoadsorbent's elemental detection was executed by energy-dispersive x-ray analysis (EDX) (VEGA-TESCAN-XMU model, Czech Republic). The X-ray diffractometer (DRON-8, Saint-Petersburg, Russia) was utilized to record the prepared samples' X-ray diffraction (XRD) pattern. The morphology of the samples was perused by the Hitachi S-5200 apparatus, Japan, and ZEISS SIGMA, Germany, for the field emission scanning electron microscope (FESEM) investigation. The study of the sample's magnetic behavior was accomplished by VSM analysis (Meghnatis, daneshpajooh Kashan, Iran). Also, the BAHR-STA 504 apparatus, Germany, was employed to investigate the change in the weight of the samples with temperature enhancement by thermogravimetric analysis (TGA) under an argon atmosphere. The samples' specific surface area, pore width, and pore volume were determined by the Brunauer-Emmett-Teller (BET, Micrometics ASAP2020, USA). The samples' surface charge was determined by zeta potential (Bruker, D8 advance, USA). Moreover, the analyses were executed to specify the properties and considerable adsorption performance of the nanoadsorbent.

Preparation of the magnetic mesoporous nanoadsorbent. Preparation of the Fe 3 O 4 magnetic na-
noparticles. Co-deposition approach was applied to prepare Fe 3 O 4 MNPs. Initially, 2.35 g of FeCl 3 .6H 2 O and 0.86 g FeCl 2 .4H 2 O, along with 40.0 mL of distilled water, were magnetically stirred under nitrogen gas flow at 80 °C for 30 min. Then, 15.0 mL of ammonia was added to the suspension to form Fe 3 O 4 MNPs. Ultimately, the black precipitate was magnetically collected and rinsed with distilled water several times; then, it was dried at 70-80 °C for 12 h.
Preparation of the pectin hydrogel. Pectin (0.75 g) was dissolved in 75.0 mL of distilled water at 50 °C for 30 min. Then, 1.5 g of CaCl 2 with 30.0 mL of distilled water was injected dropwise and stirred for 40 min. Afterward, 30.0 mL ethanol was added. 0.12 g of sodium hydroxide with 15.0 mL of distilled water was poured dropwise and mixed for 30 min, and the pectin hydrogel was formed. It was taken into the ice bath to fix its gelly nature. After forming and hardening the gel, the reaction solution was washed with 50.0 mL of ethanol and 50.0 mL of distilled water.
Preparation of the magnetic pectin hydrogel@Fe 3 O 4 . First, 0.25 g of pectin hydrogel was dissolved in 50.0 mL of distilled water for 30 min at 50 °C. Afterward, 0.15 g of CaCl 2 was slowly poured into the reaction mixture www.nature.com/scientificreports/ and stirred for 30 min at room temperature. 0.35 g of the as-prepared Fe 3 O 4 MNPs were added into 20.0 mL of distilled water and dispersed via sonication for 40 min 53 . Next, it was added gently to the reaction solution in 2-3 steps and stirred for 30 min. Eventually, 10.0 mL of methanol, 0.04 g of sodium hydroxide, and 5.0 mL of distilled water were added to the mixture and stirred for 1 h. After performing the mentioned steps, the reaction solution was washed many times with distilled water and ethanol, and a magnetic hydrogel was obtained. The obtained magnetic hydrogel was freeze-dried for 24 h. The pectin hydrogel@Fe 3 O 4 magnetic hydrogel was prepared by an ionic cross-linking procedure. The utilization of Ca 2+ divalent cation as a cross-linking agent in this preparation helped to create a 3D structure. The carboxylic acid functional groups on the pectin polymeric chain changed into COOin an alkaline peripheral caused by NaOH. Also, the hydroxyl groups became O -. These anions have electrostatic interactions with Ca 2+ . Finally, the precipitate was formed by applying ethanol and diluted NaOH solution.
Preparation of the magnetic pectin hydrogel@Fe 3  The variable parameters were perused to calculate the adsorption capacity, like pH, adsorbent amount, contact time, and initial concentration of CPF and CV. To investigate the optimum adsorption conditions, various parameters were examined. For pH adjustment in the range of 4 to 9, 0.1 M of HCl and 0.1 M of NaOH were utilized, and 5-25 mg of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent was applied in the 5-25 min contact time, and the initial concentrations of the CPF and CV were 50-400 ppm. Besides, the adsorption isotherms were perused by comparing the experimental results and Freundlich, Langmuir, Temkin, and Dubinin-Radushkevich (D-R) models. Furthermore, the adsorption kinetics were investigated via pseudo-first-order, pseudo-secondorder, and Elovich adsorption kinetic models. The concentrations of CPF and CV were determined by a UV-Vis spectrophotometer. In addition, the CPF and CV's adsorption capacity by the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent was measured via Eq. (1) and Eq. (2), respectively 6 .
where C i (mg/L) is the initial concentration and C e (mg/L) is an equilibrium concentration of CPF and CV in an aqueous solution. V (L) stands for the solution volume. And m (g) represents the pectin hydrogel@Fe 3 O 4bentonite nanoadsorbent's weight.
Regeneration and retrievability. The retrievability assessment was executed to give a view of the regeneration potential of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent and the water treatment procedure expenses. In this study, three consecutive reuse cycles in optimum conditions were carried out to observe the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent's regeneration after CPF and CV adsorption. The desorption experiment of CPF was executed as follows. After CPF adsorption by pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent in optimum conditions, i.e., 10.0 mL of solution volume, 0.005 g of adsorbent dosage, 20 min contact time, solution pH of 7, 300.0 mg/L initial concentration at an ambient temperature, the nanoadsorbent was poured into 10.0 mL ethanol and stirred at 25 °C for 3 h. Moreover, for the CV desorption experiment, after CV adsorption by pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent in optimum conditions, viz., 10.0 mL of solution volume, 0.005 g of adsorbent dosage, 15 min contact time, solution pH of 9, 300.0 mg/L initial concentration at an ambient temperature, the nanoadsorbent was added into a HCl solution (0.1 M) and stirred at 25 °C. After desorption, the nanoadsorbent was magnetically isolated from the mixture. The released CPF and CV amount were then investigated through a UV-Vis spectrophotometer. The desorption percentage (D%) was calculated using Eq. (3).
where A (mg) belongs to the desorbed contaminant quantity in the elution medium, and B (mg) stands for the adsorbed contaminant quantity by pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent.  Fig. 1, preparing this nanoadsorbent is comprised of Fe 3 O 4 MNPs preparation via a co-sedimentation approach, pectin hydrogel preparation via employing the Ca 2+ divalent cation as a cross-linker, and ultimately, bentonite as a clay material was added to the composite to enhance the surface area. As stated, due to their cross-linked 3D structure, hydrogels render high porosity leading to higher CPF and CV adsorption via the physical capturing of contaminants into the pores. On the other hand, diverse functional groups on the hydrogel, i.e., hydroxyl and carboxyl groups, result in hydrogen bonding and electrostatic interactions due to the CPF and pectin heteroatom structures. However, the main interaction between the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbet with negative surface charge (based on the data reported in Table 2) and CV cationic dye is electrostatic interactions. Therefore, the magnetic pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbet with heteroatom structures and clay substances led to an improved organic dye and organophosphorus pesticide adsorption capacity.
Characterization of the prepared magnetic pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent. Fourier transform infrared spectroscopy. Fourier transform infrared spectroscopy (FTIR) is one of the most suitable and conventional techniques for the qualitative identification of the structural functional groups of species and for determining the structure of different molecules, especially organic species. Figure 2 shows the FTIR spectra of pectin hydrogel, Fe 3 O 4 MNPs, pectin hydrogel@Fe 3 O 4 , bentonite, and pectin hydrogel@Fe 3 O 4bentonite, respectively. As depicted in the FTIR spectrum of the pectin hydrogel (Fig. 2a), the formation of the pectin hydrogel is confirmed by the presence of five distinctive absorption peaks. The broad absorption peak that arose at 3425 cm −1 is correlated with the -OH groups' stretching vibration in hydrogen bonding 54 . Two sharp peaks at 2917 cm -1 and 1425 cm -1 are due to the C-H stretching and bending vibrations, respectively. Furthermore, the stretching vibrations of -C-O-C in the glycosidic bond and carbonyl group in -COOCH 3 emerge at 1049 cm -1 and 1736 cm -1 , respectively 42,55 . As observed in Fig. 2b, the spectrum of Fe 3 O 4 has three characteristic absorption peaks. The vibration peak at 570 cm −1 is related to the bonds between iron and oxygen (Fe-O) 6 www.nature.com/scientificreports/ bonds are evident in the silicate structure and are easily identified in the FTIR spectrum by strong absorption peaks in the 1000-1100 cm −1 region. In contrast, 473 cm −1 and 524 cm −1 peaks are caused by Si-O-Al and Si-O-Si bending vibrations, respectively. In addition, the broad absorption peak at 3440 cm −1 is due to the stretching of the -OH hydrogen bond, which corresponds to the frequencies of -OH (silanol group (Si-O-H)) 61,62 . Figure 2e demonstrates the FTIR spectrum of the prepared pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent. When the pectin hydrogel@Fe 3 O 4 was functionalized with bentonite binder, the pectin hydrogel@Fe 3 O 4 became more reactive, some changes were caused in the peaks of pectin hydrogel@Fe 3 O 4, and new absorption peaks appeared. The peak that arose at 3420 cm −1 is assigned to the stretching vibration of the -OH, which was observed in all three pure substances. The adsorption peak at 1662 cm −1 is allocated to the vibrations of H-O-H in the Fe 3 O 4 MNPs. The bands observed at 2922 cm −1 and 1420 cm −1 are due to C-H stretching and bending vibrations 63 . The peak at 1043 cm −1 corresponds to the stretching vibrations of -C-O-C (glycosidic bond) of the -COOCH 3 group; all these peaks are related to pectin hydrogel. As mentioned above, the absorption bonds at 1038 cm −1 , 794 cm −1 , and 470 cm −1 are due to stretching and bending vibrations (Si-O-Si), belonging to four-dimensional SiO 4 2 , which is very evident in bentonite structure. Furthermore, the 524 cm −1 peak is caused by Al-O-Si bending vibrations, which are all specific to bentonite material. Ultimately, by combining all the distinctive absorption peaks and the bonding of materials with unique functional groups with a new chemical structure, the prepared magnetic nanoadsorbent was formed.
Energy dispersive X-ray spectroscopy. For the investigation of the sample's elements and elemental distribution in the sample, the EDX analysis was executed (Fig. 3). The O and Fe elements are assigned to the Fe 3 O 4 superparamagnetic MNPs with 38.22 W% and 61.78 W%, respectively (Fig. 3a). Based on the EDX spectrum in Field emission scanning electron microscopy. The FESEM images were provided to clarify and investigate the samples' morphology, size, shape uniformity, distribution of particle size, and possible agglomerations. Figure 4a displays the FESEM image of the prepared Fe 3 O 4 MNPs. Although the nanoparticles have a generally uniform size and spherical morphology, they are aggregated in some areas. Besides, the prepared nanoparticles rendered a narrow size distribution of ca. 40-70 nm, participating in the final adsorbent to form the Pectin hydrogel@ Fe 3 O 4 -bentonite nanoadsorbent. Based on the FESEM images in Fig. 4b,c, the bentonite components demonstrated bullet-like morphology and porous structure. The size and molecular mass of bentonite's structural phases were not alike; the material's size was different. The FESEM images of the Pectin hydrogel@Fe 3 O 4 have been taken in 700 nm and 1 μm magnifications, as shown in Fig. 4d,e, respectively. The spherical Fe 3 O 4 MNPs were well placed on the pectin hydrogel substrate, functionalized, and a magnetic hydrogel was formed. However, since the ion cross-linking of the pectin polymer strings was performed in the presence of the as-prepared The observed decrease in magnetic saturation is related to adding pectin as a natural polysaccharide and bentonite to increase the surface's active sites. Besides, these materials are non-magnetic, and reducing the magnetic saturation due to the chemical modifications and incorporation of the organic substances on the nanoadsorbent's surface seems logical. Yet, the magnetic saturation of both pectin hydrogel@Fe 3 O 4 and pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbents is sufficient to be isolated from the mixture by a magnet.
Thermogravimetric analysis. Figure 6 demonstrates the thermogravimetric behavior of the pectin hydrogel, pectin hydrogel@Fe 3 O 4 , bentonite, and pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent, and their stability over temperature rise to 800 °C with 10 °C/min stable heating rate under an air atmosphere by the thermogravimetric analysis (TGA). The thermogravimetric behavior of the pectin hydrogel (Fig. 6a) shows a 2% weight loss at 50-223 °C, indicating the evaporation and removal of the entrapped and absorbed water molecules in the pectin hydrogel's pores and surface. Further, increasing the temperature to 350 °C shows a 25% weight loss due to organic moieties' separation and thermal dissociation. The thermal behavior of the pectin hydrogel shows that the degradation of pectin hydrogel starts at ca. 250 °C and continues to 350 °C 64 (Fig. 6b), with   66 . The observed gradual weight loss can be assigned to water evaporation from the holes in this structure as well as dehydrogenation or dehydroxylation of its surface. According to the previously reported results, bentonite represents high thermal stability so that more than 91-95% of its weight has retained up to 700 °C 67 . Bentonite thermogram has demonstrated a continuous but  www.nature.com/scientificreports/ partial weight loss over enhancing the temperature due to the surface's dehydrogenation or dehydroxylation (Fig. 6c). Moreover, with a quick look at the thermogram of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent in Fig. 6d, a partial weight loss (2%) in the temperature range of 150-250 °C is attributed to evaporating and removal of the absorbed water inside and on the surface of the mesoporous structure. In addition, with the increase of temperature up to 551 °C, a weight loss of about 12% occurs because of the separating and thermal decomposition of the organic moieties of the alkyl chain that covalently bonded to bentonite 68 . Also, from the weight loss difference of the three samples, ca. 21% of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent's weight consists of the organic part. As a result, it can be concluded that pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent has high-temperature resistance.
X-ray diffraction analysis. The crystallinity investigation of the Fe 3 O 4 MNPs, pectin hydrogel@Fe 3 O 4 , and pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent was carried out by X-ray diffraction (XRD) analysis from 10° to 80° (Fig. 7). The Bragg diffraction peaks originate from (1 1 1   . In the XRD pattern of pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent, a decrease in the peak intensity at 2θ = 20° and 2θ = 36° is because of the composition and related to the chemical modifications of the magnetic pectin hydrogel@Fe 3 O 4 surface (Fig. 7e).
The N 2 adsorption-desorption isotherm. The BET analysis was carried out to explain the surface behavior and porosity of the bentonite and pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent using N 2 adsorption-desorption. As demonstrated in Fig. 8, the isotherm profiles are type-IV, representing the H4 hysteresis loop at 0.42-0.1 p/p 0 pressure. Mesoporous materials are in the type-IV isotherm profile category based on the IUPAC categorization. According to the results, the BET surface area of the bentonite was calculated 100.011 m 2 /g, demonstrating an advanced surface area compared to the reported studies (Table 1). Furthermore, the pectin hydrogel@ Fe 3 O 4 -bentonite nanoadsorbent has provided a BET surface area of 68.904 m 2 /g, which is sufficiently good. Besides, the functionalization of the pectin hydrogel, i.e., magnetization by Fe 3 O 4 MNPs, and pectin hydrogel@ Fe 3 O 4 composition with bentonite, a reduction in the pore volume and surface area were observed. Considerably, the as-prepared pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent demonstrating satisfactory textural features, enhanced porosity, and specific surface area could be contemplated as a potent nanoadsorbent in different water contaminant removal.
Zeta potential measurements. The superficial charges of the as-prepared pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent were measured by zeta potential test. Zeta potential measurements were performed once before adsorption and once after CV dye adsorption at room temperature. Due to the reported results, at pH of 3, 5, 7, and 9, the measured zeta potential of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent in deionized (DI) water were − 32.2, − 33.1, − 34.1, and − 42.8, respectively ( Table 2). The increase in the absolute value of the zeta potential value is related to the pH value increase, as the structural hydroxyl and carboxylic acid groups are deprotonated. More importantly, due to the noticeable negative surface charge of the pectin hydrogel@Fe 3 O 4bentonite nanoadsorbent, the nanoadsorbent particles do not aggregate at various pH values. By investigating the zeta potential measurements after CV dye adsorption, it is deduced that at pH of 3, 5, 7, and 9, the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent's zeta potential in the CV solution was − 32.8, − 34.3, − 35.9, and − 36.2, respectively. Since the CV dye has a cationic structure, it has robust electrostatic interactions with negatively charged pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent.  Table 1. Surface area, pore volume, and pore size of bentonite and pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent. a The surface area parameter was acquired via BET analysis. b The pore volume and pore size parameters were acquired via BJH analysis.

Sample
Surface area a (m 2 .g −1 ) Pore volume b (cm 3 76,77 . Further, the optimization of these effective parameters upgrades the contaminant removal. The pH effect on the CPF and CV adsorption on the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent is presented in Fig. 9a. Through the solution pH increase from 4 to 7, the adsorption capacity incremented from ca. 78.6 and 80.2 mg/g to 91.9 and 88.3 mg/g for CV and CPF, respectively. However, by elevating the pH to 8, the adsorption capacity of CPF was reduced to 82.6 mg/g, and the adsorption capacity of CV climbed to 96.5 mg/g. At acidic pH (i.e., pH 5), the CPF adsorption capacity declined and reached 80.6 mg/g because of the competition between the hydroxyl groups of the nanoadsorbent and excess H + around the adsorbent for unoccupied adsorption sites. On the other hand, the lower CPF adsorption capacity at high pH amounts (i.e., 9) can be assigned to the electrostatic repulsion between extra OH anions and the nanoadsorbent's OH groups 78,79 . Also, the leading bondings are the hydrogen bonding between the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent and CPF molecules with a non-ionic structure as an organophosphorus pesticide. Thus, the solution pH of 7 provides the minimum amount of cationic and anionic substances, diminishing their impact on the pollutant and nanoadsorbent active sites' interactions. In this regard, for the CV with a positively-charged structure, the driving forces for its adsorption on the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent with negative surface charge are the electrostatic attraction, which led to an enhanced adsorption capacity at higher solution pH. This redult was consistent with Eltaweil et al. study which evinced the proper effect of the neutral and alkaline environment for adsorptive elimination of detrimental cationic dyes 80 . According to the experimental results, an ideal solution pH for CPF and CV was 7 and 8, respectively, representing the largest adsorption capacity.
Adsorbent dosage. The relation between the nanoadsorbents amount and the related adsorption capacity for CPF and CV was investigated using various pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent amounts at optimum pH. Based on the results in Fig. 9b, with an increase in the adsorbent dosage from 0.005 g to 0.025 g, the adsorption capacity of the magnetic nanoadsorbent for CPF and CV decreased from ca. 170.2 mg/g and 192.3 mg/g to 37.5 mg/g and 38.1 mg/g, respectively. The availability of CPF and CV with a large amount to be adsorbed by pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent corresponds to the enhanced adsorption capability at lower adsorbent dosages, which is associated with the increased contaminant amount available for the nanoadsorbent. Therefore, 0.005 g of pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent was demonstrated to have the most efficient adsorbent dosage for further studies.
Contact time. The contact time impact on the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent to adsorb CPF and CV was investigated from 5 to 25 min at optimum pH and adsorbent dosage. In this regard, the CPF adsorption capacity demonstrates an increasing trend of ca. 200.0 mg/g by the progress of the time to 15 min (Fig. 9c). However, the adsorption capacity decreases slightly to 20 min and has an approximately steady trend to 25 min. The adsorption capacity of CV shows improvement to ca. 178.8 mg/g after 20 min contact time and gradually reduces to 171.6 mg/g after 25 min. Thus, an optimum contact time for CPF and CV was ascertained as 15 min and 20 min, respectively. The increasing trend at the initiation of the adsorption reaction is related to the numerous unoccupied active sites of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent available for interaction with CPF organophosphorus pesticide and CV organic dye, resulting in a quick mass transfer. Efficient interactions among the nanoadsorbent's functional groups and CPF pesticide and CV dye are progressive until it reaches the maximum equilibrium adsorption capacity after passing 15 min and 20 min of the reaction process.
In contrast, no progress in adsorption capacity was observed after the optimum contact time, which is assigned to occupying the nanoadsorbent's active sites after reaching the maximum equilibrium adsorption capacity.
Initial pollutant concentration. The employed contaminant concentration was the last parameter debated in this section to determine its effect on the adsorption capacity of the pectin hydrogel@Fe 3 O 4 -bentonite nanoad- www.nature.com/scientificreports/ sorbent through regulating the CPD and CV initial concentration from 50 to 400 mg/L at optimum pH, adsorbent dosage, and contact time. Based on Fig. 9d, increasing the CPF and CV initial concentrations from 50 to 400 mg/L enhanced the adsorption capacity to 703.8 mg/g and 665.4 mg/g, respectively. Certainly, by increasing the CPF and CV initial concentration at a constant adsorbent dosage, the CPF organophosphorus pesticide and CV dye adsorbate amount to pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent ratio was enhanced, leading to an improved adsorption capacity.

Adsorption isotherm and adsorption kinetics studies.
The adsorption isotherms were perused to investigate the interactions between the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent and CPF pesticide and CV dye. Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R)'s isotherms have been investigated to determine equilibrium adsorption isotherms and compute the highest amount of the adsorption capacity 81 . Langmuir isotherm (Eq. (4)) is a model to describe the one-layer contaminant adsorption on the adsorbent's surface. In this case, all of the adsorbent's superficial sites have similar energy and affinity to interact effectively with contaminants, leading to homogeneous adsorption 17 . Indeed, the maximum adsorption capacity is considered when a complete monolayer of the contaminants is formed on the adsorbent's superficial. On the opposite, the Freundlich isotherm (Eq. (5)) defines as a model to describe multilayer adsorption of pollutants on the heterogeneous surface of the adsorbent. Temkin isotherm (Eq. (6)) defines how the adsorbate substances and the adsorbent's system have interacted and the adsorption procedure's bonding energies. In this isotherm, the adsorption energy between the adsorbent's covered surface and adsorbate diminished based on the descending linear pattern. The Temkin model represents the adsorption procedure's free energy as an adsorbent's surface coating function 82 . The Dubinin-Radushkevich (D-R) model (Eqs. (7), (8)) is an empirical isotherm utilized to describe the adsorption procedure based on the pore-filling mechanism. The wide applicability of this isotherm is to represent the adsorption mechanism executed onto homogeneous and heterogeneous surfaces 83 . The equa- where C e (mg/L) belongs to the equilibrium concentration of CPF and CV, Q e (mg/g) is the equilibrium adsorption capacity, and Q max (mg/g) represents the highest adsorption capacity. K L (L/mg) and K F (L/mg) are constants in Langmuir and Freundlich isotherms computed from the plot between C e /Q e and C e , and between log Q e and log C e , respectively. In the case of n > 1, the CPF and CV's adsorption at high concentrations on the adsorption surface is favorable 17 . For the Temkin isotherm (Eq. (6)), R represents the universal gas constant, T (K) stands for the temperature, b T is ascribed to the adsorption heat, and K T (L.mg −1 ) is the constant of the Temkin model. In Eqs. (7), (8) for Dubinin-Radushkevich (D-R) isotherm model, q s (mg P/g) is adsorption capacity-related Dubinin-Radushkevich (D-R)'s constant, K DR (mol 2 /kJ 2 ) stands for adsorption's average free energy, R (J/mol K) stands for the gas constant, and T (K) is the temperature. According to the charts of the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms in Fig. 10a-d, respectively, and based on the data presented in informative Table 3, obtained from Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms, Freundlich isotherm well matches the experimental information compared to Langmuir, Temkin, and Dubinin-Radushkevich (D-R) isotherms for CPF and CV. The adsorption kinetics linear plots and the computed CPF and CV contaminants' parameters were exhibited in Fig. 10e-

Recyclability of pectin hydrogel@Fe 3 O 4 -bentonite magnetic mesoporous nanoadsorbent in adsorption process and interfaces assessment.
The retrievability of nanoadsorbent is highly significant for application in industrial facets 17 . According to the attained results, as shown in Fig. 11, the recyclability of pectin hydrogel@Fe 3 O 4 -bentonite magnetic mesoporous nanoadsorbent was accomplished for three consecutive adsorption-desorption cycles. To peruse the desorption, the magnetic nanoadsorbents were added into the HCl solution with 0.1 M concentration and shaken for desorption at room temperature separately to dissociate the nanoadsorbent-CV and nanoadsorbent-CPF bonds. Then, the magnetic nanoadsorbents were isolated by a magnet, rinsed with distilled water, and dried in an oven at 60 °C. Due to the consecutive reusability cycles in three runs, no substantial decrease was detected in the CPF and CV adsorption capacity. In this line, the pectin hydrogel@Fe 3 O 4 -bentonite magnetic mesoporous nanoadsorbent is a great system for removing CPF and CV, and it can be conveniently collected without any considerable diminish in adsorption efficiency. After the CPF and CV adsorption at optimum conditions, the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent was filtered, and the ICP test was taken from the filtrate. Based on the ICP results, the Fe 3+ and Ca 2+ ions concentration released into the filtrate solution was measured to be 3.21 ppm and 2.09 ppm, respectively. This meager leaching amount is ascribed to the incomplete magnetic nanoadsorbent separation from the reaction mixture after completing the adsorption with a magnet. Hence, the released ions to the filtrate solution were so few, indicating the nanoadsorbent's stability and applicability to be recycled for three successive cycles without any loss in adsorption efficiency. Proposed mechanism of adsorption. As previously stated, the pectin hydrogel@Fe 3 O 4 -bentonite magnetic nanoadsorbent has a prominent role in organic dye and pesticide elimination according to its enhanced  www.nature.com/scientificreports/ surface area and mesoporosity. Additionally, abundant hydroxyl and carboxyl functional groups of the pectin hydrogel tend to provide a hydrogen bond network with CV and CPF. The electrostatic interaction between CPF and the magnetic nanoadsorbent occurs due to their heteroatom structures. It should be considered that based on the anionic nature of the prepared nanoadsorbent in different acidic, neutral, and alkaline solution pH, as shown in Table 2, and the cationic charge of the CV, the leading interaction between CV and the prepared magnetic nanoadsorbet would be an electrostatic attraction (Fig. 12). Notably, the highest adsorption efficiency related to the most effective electrostatic interaction between the nanoadsorbent and CV occurred at pH = 8 because, in the acidic pH, the produced H + in the solution acts as a competing species with positively-charged CV for proper binding sites on the surface of the prepared adsorbent; therefore, in acidic pH, the adsorption efficiency reduces. Furthermore, with a quick glance at the remarkable porous structure of the cross-linked pectin hydrogel, plausible physicochemical adsorption, and BET surface area of the nanoadsorbent (68.904 m 2 /g), which mainly arose from the bentonite addition to the nanocomposite, it can be deduced that CPF and CV can be physically trapped in the nanoadsorbent structure. Overall, numerous pores, OH and COOH groups, and  www.nature.com/scientificreports/ also the heteroatom structure of the nanoadsorbent have promoted the affinity and adsorption capacity toward CPF and CV contaminants.

Conclusion
In this work, the prepared pectin hydrogel@Fe 3 O 4 -bentonite magnetic nanoadsorbent demonstrated an excellent magnetism, due to the presence of the Fe 3 O 4 MNPs in its structure. The magnetic nanoadsorbent was characterized via different analatical approaches, such as FTIR, EDX, FESEM, XRD, TGA, BET, and VSM to find out the precise structural properties of the prepared nanoadsorbent. The adsorption efficiency of the organophosphorus CPF and CV organic dye in an aqueous medium was calculated. The CPF and CV adsorption capacity was 833.333 mg/g and 909.091 mg/g in optimum conditions, respectively. TGA curve displayed ca. 15% weight loss to 800 °C, indicating the enhanced thermal stability of the pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent. Besides, XRD patterns clearly show the immobilization and uniformity in the distribution of Fe 3 O 4 MNPs in the nanoadsorbents, enhancing the crystallinity of the structure even after adding bentonite. Due to the VSM results,  www.nature.com/scientificreports/ the magnetic saturation of the nanoadsorbent was 20.53 emu/g without magnetic Remanence and coercivity, and it represented the superparamagnetic properties. The FESEM images showed the uniformity in size and shape of the prepared Fe 3 O 4 MNPs throughout the cross-linked structure of pectin hydrogel. Also, after bentonite addition, smoothing the pore's surface and the formation of the layered structure in FESEM images are evident. The pectin hydrogel@Fe 3 O 4 -bentonite nanoadsorbent was introduced as an effective adsorption system for eliminating CPF and CV from aqueous media based on the enhanced surface area, numerous active interaction sites, and effective interactions with contaminants. Generally, the prepared nanoadsorbent was an appropriate material to scale up and industrialization regarding the convenient isolation from the reaction, preparation from affordable substances, efficient binding to the pollutants, and maintenance the structural stability after three consecutive retrievability cycles with mo remarkable decrease in the adsorption yield.

Data availability
All data generated or analysed during this study are included in this submitted article (and its Supplementary  Information files).